1. Field of the Invention
The present invention relates to an optical information recording and regenerating apparatus which optically records and regenerates information by using a beam of light emitted from a laser light source and, more particularly, to a structure of a regenerating system which regulates the amplitude of a regenerated signal.
2. Description of the Related Art
There has extensively been used an optical information recording and regenerating apparatus which is designed to record information by applying a beam of light from a laser light source to an optical recording medium such as an optical disk and to regenerate the information by receiving the reflected light from the medium.
For example, in an optical disk unit which employs an optical disk as the medium, there are some cases where the level of a regenerated signal obtained by the regenerating process varies greatly because the replaceable medium tends to cause significant variations in reflectance of the medium or because the optical efficiency varies as the optical system becomes soiled due to dust or the like.
In a pit-position recording type apparatus wherein the position of the peak of a regenerated signal corresponds to "1" of data, as illustrated in FIG. 1, for instance, the regenerated signal obtained by regeneration is sliced at an appropriate voltage level (slice level) to generate a gate signal. The zero-cross point of the differential signal of the regenerated signal during a period in which the gate signal stays ON (hereinafter referred to simply as "the differential signal") is detected. The signal based on the AND (logical product) of the zero-cross signal which reverses at each zero-cross point and the gate signal is turned into a binary-coded signal. The regenerated data is produced according to the binary-coded signal.
In such an apparatus, when the amplitude level of a regenerated signal changes with the slice level fixed, the peak of the regenerated signal sometimes may not exceed the slice level when the amplitude is small, presenting such problems as no gate signal generated or deteriorated accuracy in detecting a sector mark for determining the start position of a sector. To prevent such errors from occurring at the time of regeneration, an automatic gain control circuit (AGC circuit) is used to control the amplitude of a regenerated signal.
For instance, Japanese Patent Laid-Open No. 58-73022 has disclosed an apparatus which is designed to carry out AGC so as to provide uniform amplitude of "VFO signal" in a regenerated signal in the preformat area of a medium, thereby achieving uniform amplitude of the regenerated signal. Further, Japanese Patent Laid-Open No. 56-94515 has disclosed an apparatus which is designed to carry out AGC on the mean value of the differential signals of the preamble signals which are obtained by reading the preamble data from a medium, thus achieving uniform amplitude of the regenerated signal. Furthermore, Japanese Patent Laid-Open No. 64-25356 has disclosed an apparatus which is designed to carry out AGC independently on the regenerated signal in the preformat area and that in the data area of a medium.
The operating methods based on the AGC carried out on regenerated signals in the conventional apparatuses described above may cause the problems stated below as the modulation transfer function (hereinafter referred to as MTF which means the signal resolution in the field of optical information recording and regeneration) becomes smaller with increasing density of data.
The following describes the case wherein AGC is performed to make uniform the amplitude of the VFO signal which is regenerated from the VFO area in the preformat area as in the case of the apparatus disclosed under Japanese Patent Laid-Open No. 58-73022. The VFO signal is repeated at the shortest data interval, i.e. 3T, of (2, 7) code which is used as a recording code. Therefore, carrying out AGC on the VFO signal eventually means carrying out AGC on the smallest signal amplitude in the regenerated signal. The minimum value of MTF is approximately 20%, while the maximum amplitude of the regenerated signal is approximately five times as large as the VFO signal; therefore, any attempt to control the VFO signal to an appropriate level may cause the regenerated signal to be saturated in the data area or the like. On the other hand, if a target value for the AGC output level is set to prevent the saturation of the signal even when the MTF reduces down to 20%, then the output amplitude level of the AGC circuit becomes extremely low when the MTF is large.
In addition, in the case of an optical disk unit, a regenerated signal contains a DC component; therefore, it is subjected to AC coupling in a regenerating system before it is transmitted. This causes the transient of the regenerated signal as illustrated in FIG. 2 at the beginning of the preformat area. As a result, the amplitude of the regenerated signal which is measured from the ground level (GND) apparently becomes large. Hence, when implementing AGC on such a regenerated signal, unless a correct timing is selected to initiate AGC, AGC is actuated at the signal level which is involved in the aforesaid transient. As a result, it may erroneously be determined that the signal amplitude is large, leading to an desirably small output amplitude level of the AGC circuit.
According to the apparatus disclosed under Japanese Patent Laid-Open No. 56-94515, AGC is carried out on the mean value of differential signals; therefore, the aforesaid inconvenience in the apparatus disclosed under Japanese Patent Laid-Open No. 58-73022, which determines the output amplitude level of AGC only according to the amplitude of the VFO signal can be improved. The apparatus disclosed under Japanese Patent Laid-Open No. 56-94515, however, still may cause the signal output amplitude to be saturated in the data area because, in the preformat area of the optical disk, for example, 28 bytes out of 52 bytes are assigned to the VFO area, that is, the VFO area occupies the majority of the preformat area. Therefore, the signal output amplitude in the data area may still be saturated after differentiation if the MTF of a regenerated signal is small because the amplitude in the VFO area is small.